The rain has been caused by remnants of ex-tropical cyclone Debbie clashing with a cold front.

People living in and around South Murwillumbah, Condong, Tumbulgum, Lismore, Chinderah, Kingscliff, Fingal Head and Bilambil were given evacuation orders by the State Emergency Service as nearby rivers began to break their banks.

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A young boy looks out over the swollen Tweed River in Murwillumbah after heavy rain.Credit:Jason O'Brien

"The NSW State Emergency Service is directing residents to evacuate immediately where safe transit exists and they are able to do so," an evacuation order issued about 4pm said.

"Do not delay your evacuation. Roads will be congested or closed. You could become trapped and need rescue."

The SES emphasised that the order was not a warning: "Evacuations from these areas are mandatory. You must leave NOW. Evacuation Order is in place."

Mark Morrow, the Deputy Acting Commissioner for the State Emergency Service, said the biggest concern was the Wilsons River at Lismore.

Kids paddle in a kayak as floodwaters inundate their yard in Murwillumbah.Credit:Getty Images

"That's risen so quickly, in fact, throughout the course of the afternoon that we've gone straight to an evacuation order for around 6000 residents that are subject to that order," Mr Morrow told ABC News 24. "In addition to that we have evacuation warnings in place."

Residents of South Murwillumbah had earlier been warned of a possible evacuation.

Semi-submerged homes along the Tweed River at Murwillumbah.Credit:Getty Images

Mr Morrow said around 30 flood rescues have been carried out in the last 24 hours, including five on Thursday afternoon.

Residents of South Murwillumbah and nearby suburbs have been ordered to evacuate.Credit:Getty Images

"It's very dangerous conditions out there over the next 24 hours, so please act responsibly."

Mr Morrow said there will be "major" flooding in Lismore, with Southern Cross University set up as the evacuation centre.

The Wilsons River is expected to reach 9.7 metres late on Thursday night and peak at 11 metres early on Friday morning, after the Wilsons River valley received up to 430 millimetres of rain in 22 hours.

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Residents were told to bring important documents, spare clothing and sleeping bags, to check on their neighbours, and to turn off their electricity and gas before leaving their homes.

The Murwillumbah evacuation centre is at the Kings Sacred Heart hall on Murwillumbah Street.

Residents of Tweed Heads have also been warned they may need to evacuate.

"Further rises are possible with rainfall forecast overnight tonight," the Bureau said in a flood warning.

Almost 400 millimetres of rain has fallen in just 24 hours over the Tweed River valley.

In a severe weather warning, the Bureau of Meteorology said a humid tropical air mass is expected to cross the far north coast on Thursday night and intensify, before moving offshore on Friday afternoon.

"Very heavy rain is occurring over the northern parts of the coast and is expected to continue into early Friday," the bureau said. "Heavy rain which may lead to flash flooding is also possible over northern ranges tonight."

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More than 100mm of rain is expected to fall over parts of the northern rivers from 10pm to 7am, with thunderstorms to possibly "amplify rainfall" up to 150mm in some areas.